Silvia Ma Lu , Dazhi Yang, Martha Anderson, Sebastian Zainali, Bengt Stridh, Anders Avelin, Pietro Elia Campana

Photosynthetically active radiation is a key parameter for modelling the photosynthetic behaviour of plants in response to sunlight and, subsequently, for determining crop yield. Separating photosynthetically active radiation into direct and diffuse components is of significance to agrivoltaic systems, which combine solar energy conversion and agricultural farming on the same portion of land. Placing photovoltaic on agricultural land results in varying shading conditions throughout the day and seasons, producing a higher contribution of incident diffuse radiation to the crops beneath the system in these shaded regions. Additionally, photosynthesis is more efficient under conditions of diffuse radiation than direct radiation per unit of total photosynthetically active radiation. This study introduces a new separation model capable of accurately estimating the diffuse component from the global photosynthetically active radiation and conveniently retrievable meteorological parameters. The model builds upon one of the currently most performing separation models for decomposing global solar radiation into diffuse radiation, the YANG2 model. Four new predictors, found relevant in the literature to influence diffuse photosynthetically active radiation, are added: optical thickness, vapour pressure deficit, aerosol optical depth, and surface albedo. The proposed model has been calibrated, tested, and validated at three sites in Sweden with latitudes above 58° N, outperforming four other models in all examined locations, with R2 values greater than 0.90. To alleviate the scarcity of photosynthetically active radiation studies in high-latitude regions, the seasonal trends and variation of the various radiation components are analysed. To demonstrate the applicability of the developed model, the location of Sweden's first agrivoltaic system is used to assess a variety of cases that take into account the various data availability that is representative of current and future agrivoltaic systems. In cases where in-situ measurements of diffuse photosynthetically active radiation are not available, the results indicate that the model can be utilised and adjusted for adjacent stations. Utilising predictor values derived from satellite data is an alternative method, but the spatial resolution must be considered with caution.